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Re: [Discuss-gnuradio] Works with GR 3.6, breaks with 3.7

From: John Malsbury
Subject: Re: [Discuss-gnuradio] Works with GR 3.6, breaks with 3.7
Date: Fri, 17 Oct 2014 12:39:31 -0700

The good news is.  the GFSK block doesn't do anything specific to Gaussian FSK.  So I guess tha tall works out =)

You're outputs might be more illustrative if you rebuild it in from the blocks actually inside the GFSK demod block:

quad demod > clock recovery -> slicer

On Fri, Oct 17, 2014 at 12:33 PM, Luke Berndt <address@hidden> wrote:
It actually looks like the control channel for Motorola SmartNet is FSK: "On the control (data) channel the base station transmits 84 bits frames at 3600 bit/s with direct frequency modulation of the carrier using Frequency Shift Keying (FSK). "

I will at least see if I can get something that looks right out the other side. Would it be best to use one of the GFSK examples in gr-digital?

On Fri, Oct 17, 2014 at 10:37 AM, John Malsbury <address@hidden> wrote:
Also, my understanding for the PLL blocks were that they were ideal for "strong carrier" signals like AM.  When I say strong carrier i mean a signal that has an obvious carrier which isn't "hidden" under modulation..

Anyway, the GMSK block might be a good place to start.


On Fri, Oct 17, 2014 at 7:35 AM, John Malsbury <address@hidden> wrote:
It doesn't have frequency correction - I can probably follow up with some ideas on how to implement that part.  But the GMSK demod might do OK for you initially.  It doesn't do anything intelligent to deal with the data shaping - its just a non-coherent slicer.

If you want to design in GRC but keep your top-level application as is, you can use a hier block.  You can also use a command line parameter or other mechanism to select your demodulator at start-time for easy comparisons. 

[typed one handed.. my daughter has my other and]


On Fri, Oct 17, 2014 at 7:08 AM, Luke Berndt <address@hidden> wrote:
Thanks for looking into it! To be honest, I am not really good at RF. I based my code off the python code in gr-smartnet. The fsk-demod python file is here: https://github.com/bistromath/gr-smartnet/blob/master/src/python/fsk_demod.py

It is quite possible that it just happened to work because of an error that got patched in Gr 3.7.

Are there some good examples for GMSK/FSK demodulation that I could borrow from instead?

Recreating this in GRC sounds like a great idea so I can scope along the way. I will give that a try next.

Thanks again for the pointers, fresh eyes are really helpful when you have been staring at it for so long.

 - Luke

On Fri, Oct 17, 2014 at 8:18 AM, Martin Braun <address@hidden> wrote:
On 10/17/2014 08:28 AM, John Malsbury wrote:
> Also also, is the Band-Edge FLL ideal for GMSK?  My possibly, incorrect
> understanding of that block is that it was more ideal for PAM with
> common RRC.

For the record: It might work coincidentally because of the rolloff-y
shape of GMSK, but it's derived for and designed for common PAM/RRC, as
John says and I wouldn't recommend it for anything else.
To look up the math, I suggest Harris' works.

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